The ocean circulation is driven by the transfer of heat, water, and momentum across the air-sea interface. Heat transfer is accomplished through (1) latent heat flux, (2) sensible heat flux, and (3) radiation. Water transfer is accomplished through evaporation and precipitation. Momentum transfer takes place through the frictional effects of winds on the ocean surface.

If we imagine that ocean circulation is only responding simply to winds (ignoring that the ocean is in rotation), we would expect simple gyres to form in each ocean basins in response to the tropical easterly trades and midlatitude westerlies with the north-south branches along the coastlines to conserve mass. The gyre centers would be at about 30 N and 30 S in the center of the basin, and water would pile up where the eastward and westward moving branches approached land (see Fig 5-1 and the first row in the figure below) 

and it the surface would be low in the opposite corners. Now if we account for the influence of Earth's rotation we create a net transport of the surface water at a 90 deg angle to the wind, we see that the water piles up in the center of the gyre, instead of at the corners. However this still ignores conservation of vorticity. The middle row of pictures would be accurate if the Earth were a rotating disk and not a sphere (hence the coriolis force would be constant). But on a sphere, the coriolis force increases with latitude, and this causes the branch of the gyre that is near western boundaries to be concentrated in what is known as a western boundary current (lowest row).

Like the geostrophic wind, the geostrophic current flows perpendicular to the two forces that balance to allow this steady flow. The forces are the pressure gradient force and the coriolis force (see figure at right). The geostrophic current would flow around the water that piles up in the middle of the gyre, except friction slows it down a little, which makes the actual current flow diverge outward a little.

The figure on the left shows how Ekman transport causes coastal upwelling. The "coriolis effect" in the figure on the left is our Ekman transport. In the upper panel, the Ekman transport pulls water away from the coast. This water is then replaced by upwelling deeper water, which is colder. Locations in the northern hemisphere with persistent northerly winds just offshore have cold water at the coast, like in our state and in Oregon and California. The picture immediate below shows how Ekman transport due to the easterly trade winds causes upwelling along the equator.